This came up because we have a character on one of the Star Trek shows that's now airing that is non-binary. Somebody said that this is the first time we've had a non-binary Trek character, but then somebody said: "Hey, what about that species with 3 genders?", and yadda yadda now everyone's wondering what the term "non-binary" would mean in all sorts of strange situations you can encounter on a sci-fi show (and maybe out there in our universe too)
The thing is that the alien societies depicted on Star Trek are anthropomorhic to an extent as well; these alien races are supposed to mirror human characteristics so we can relate to them and learn whatever lesson and so on.. I mean take any random species from Star Trek. They wear pants, they have jobs, they eat dinners and breakfasts, they poop in a toilet, they drink rum, they're human! So it seems it should be able to extend this concept to hypothetical scenarios that might appear on the show (i.e. an alien species with 4 genders or what have you), unless it's a concept that only applies to humans? Which I've never seen it described as such (but I haven't looked very far either)
If it's basically a rejection of traditional gender roles like you say, and that's the "real" definition of what it means, then I get it. But if that's the case why won't people just say that? I also get that if there were really aliens with 4 genders out there, our social studies people would come up with new terms for all their social behaviours, and this whole question I am asking would be an entire field of study instead of one question on some random forum.